1. Field of the Invention
The invention relates to a method for determining the characteristics of a protective device to an electrostatic discharge event based upon kinetic energy levels transferred during an air discharge simulation of the electrostatic discharge event and, more particularly, to a method for testing the susceptibility of an assembly to an electrostatic discharge event based upon the characteristics of the protective device.
2. Description of Related Art
Electrostatic discharge events occur when there is a dielectric breakdown between two oppositely charged pockets of energy. For example, triboelectricity is the generation of charges when rubbing together and quickly separating two dissimilar materials. Triboelectricity can charge an insulated capacitor into a high voltage source. This charged capacitor may then be discharge to any conductor, thereby resulting in an electrostatic discharge event. While an assembly threatening, electrostatic discharge event may originate from any one of numerous sources, one very common source is a discharge from a metal object such as a key, tool, bracelet or ring held by or worn on the hands of a triboelectrically charged individual. Such discharges can readily generate a current wave having a high amplitude initial spike having a duration of about 1 to 4 nanoseconds and a peak current in the range of 10 to 30 A with a rise time of less than 1 nanosecond.
Other electrostatic discharge events, particularly those affected by certain parameters which are related to the arc formation and energy transfer which characterizes an electrostatic discharge event, can represent an even more serious threat to an assembly. These parameters, which include dielectric strength, humidity, atmospheric pressure, and speed at which the electric field gradient forms, can combine to give a threat which ranges from 200 picoseconds to 10 nanoseconds rise times, 1 amp to 100 amps peak current, microjoule to millijoule energy levels and up to 200 nanoseconds time duration. Thus, electrostatic discharge events, particularly those in the ranges herein identified, represent a significant threat to assemblies, particularly, those assemblies which include low voltage, discrete electronic devices such as microprocessors. Such assemblies would be severely damaged or destroyed if subjected to an electrostatic discharge event of such magnitude. In recent years, the need to protect such assemblies has become even more pressing with the increased use of CMOS and other low power semiconductor components. This recognition of the threat which electrostatic discharge events pose to the low power devices popular today has motivated increasing levels of interest in this phenomenon.
Considerable effort has been applied to the study of electrostatic discharge events, in particular to the study of the susceptibility of electronic components to various electrostatic discharge events. However, many factors have complicated the effective study of how to protect components from susceptibility to electrostatic discharge events. For example, the highly transient nature of electrostatic discharge events, the highly variable nature of the air through which an electrostatic event is discharged, and the differences between electrostatic discharge events caused by a simulator and by a triboelectrically charged person have seriously limited attempts to standardize electrostatic discharge event test techniques as well as attempts to establish the level of electrostatic discharge threat protection which should be provided for a given component.
While accurate modeling by electrostatic discharge event simulators of the threat produced by an electrostatic discharge event has improved in recent years (See, Richman, "An ESD Circuit Model with Initial Spikes to Duplicate Discharges from Hands with Metal Objects", EMC Technology (1985) and Richman and Tasker, "ESD Testing: The Interface Between Simulator and Equipment Under Test", Proceedings of the 6th EMC Symposium, Zurich, Switzerland, 1985), considerable difficulties in determining component susceptibility to electrostatic discharge events remain, particularly in view of the difficulties resultant from the fact that most electrostatic discharge events are air discharges. For example, Sperber and Minnich, "Test Procedure and Specification for Component Susceptibility to Electrostatic Discharges", IEEE International EMC Symposium, Seattle, Wash., 1988, recommends that the test procedures to determine component susceptibility to electrostatic discharges specify a fixed-gap method of discharge to the device under test (or "DUT") would provide consistency in test results. However, the method disclosed by Sperber and Minnich relied upon the discharge voltage for the ESD simulator measured by placing a high impedance probe across the ESD output to make evaluations.